From what little experimenting I've done with Tachi: Jinpu, I've found that the damage is far too variable to effectively DD (outside of solo/tank). The 10k is likely Apoc proc-ing on both hits. Either that or you have some silly MAB atma I've never heard of that my BLM would be very interested in.
I like dat damage, though. I got 2-3k averages on standard mobs.